(1) Field of the Invention
The present invention relates to a filter box, to a fuel circuit for feeding fuel to an engine and provided with the filter box, and to an aircraft including the fuel circuit.
The invention thus lies in the technical field of aircraft, and more specifically of rotorcraft.
(2) Description of Related Art
Rotorcraft are aircraft presenting the particular feature of being fitted with at least one rotary wing providing them at least with lift, and possibly also with propulsion and/or the ability to maneuver in flight. Such a rotary wing is driven in rotation by a power plant comprising one or more engines.
Under such circumstances, a fuel circuit of an aircraft may comprise a fuel storage tank that may be installed in the bottom section of a rotorcraft, for example.
Conventionally, a fuel circuit comprises one main tank per engine, each main tank being in fluid flow communication with an engine. Such a main tank is sometimes referred to by the person skilled in the art as a “feeder”. Fuel is then drawn from the main tank in order to be taken to the associated engine. The fuel may be conveyed from the main tank to an engine by a booster pump arranged in the main tank and/or by a suction pump of an engine.
The fuel circuit may also include at least one secondary tank in fluid flow communication with at least one main tank.
For example, the main tank may be arranged inside a secondary tank, or it may be arranged beside a secondary tank.
The main tank is thus formed by an enclosure of volume suitable for enabling an engine to be fed with fuel for an operating duration that is defined by certification regulations.
On a twin-engine aircraft, the fuel circuit may comprise two main tanks and two secondary tanks, each secondary tank being in fluid flow communication with each main tank.
Furthermore, a main tank may be fed with fuel from a secondary tank via a transfer system making use of a transfer pump housed inside the main tank. The transfer system then has an ejector placed inside a secondary tank for conveying fuel from the secondary tank to the main tank.
Thus, a transfer pump arranged in a main tank is connected by a pipe to at least one ejector arranged in a secondary tank. The ejector is then connected to the main tank via another pipe.
By using a flow of a primary stream of fuel generated by the transfer pump inside the transfer system and taken from the main tank, such an ejector serves to capture a secondary stream of fuel in the secondary tank. The secondary stream of fuel is then delivered together with the primary stream out from the ejector into the main tank via a pipe provided for this purpose. Since the capacity of the main tank is limited, the main tank may be fitted with an overflow device for restoring any excess fuel to a secondary tank as an overflow.
Given the proximity between a main tank and a secondary tank, and given the small head losses induced by the transfer system, the transfer pump may be a low pressure pump; as an indication, it may deliver a pressure of the order of less than 200 millibars (mB), and more commonly less than 100 mB. The transfer pump may in particular be driven by electricity taken from the on-board electrical power supply network of the aircraft.
Furthermore, in general terms, an ejector comprises an upstream nozzle for admitting a primary fluid stream and leading to a main channel having a constricted portion. A secondary channel for admitting a secondary fluid stream opens out into the main channel upstream from the nozzle. The flow of the main stream inside the main channel acts by suction to capture the secondary stream of fluid through the secondary channel. The secondary stream and the primary stream mix together upstream from the constricted portion, and then the overall fluid stream resulting from this mixing is discharged from the ejector downstream from the constricted portion. The concepts of upstream and downstream should be considered relative to the flow direction of the various streams through the ejector.
The ejector then operates by the Venturi effect in order to generate a stream of fuel going from a secondary tank to a main tank.
Furthermore, the equipment of a transfer system usually includes filter grids for preventing polluting material being transferred to an engine in order to prevent the equipment becoming clogged. Booster pumps, ejectors, and transfer pumps thus sometimes include filter grids for filtering out pollution. Such grids are sometimes referred to as “strainers” or “screens”.
These grids are mounted on the equipment in order to filter fuel.
Grids are inspected during maintenance operations in order for them to be cleaned in the event of becoming clogged by pollution. Nevertheless, grids are difficult to access.
On a rotorcraft having tanks arranged in the bottom section of the rotorcraft, each tank is of low height. Furthermore, the tanks are difficult to access. Consequently, an operator may need to remove an entire tank in order to access a grid of an ejector, for example.
Periodically cleaning filter grids in equipment of a fuel circuit can thus lead to the fuel circuit being emptied, then to the tanks being opened, and then to the equipment being disassembled. That method requires maintenance operations that are burdensome, lengthy, and expensive.
Document U.S. Pat. No. 3,275,061 (Boeing Co.) describes a feed ejector installed in a fuel feed line for an engine of an aircraft, and more particularly of an airplane.
The pressure of the fuel in a feed line is then increased by the ejector.
Certain aircraft may be provided with ejectors for maximizing the pressure of a stream of fuel before it is admitted into an engine.
For reasons of safety, that technical option may be difficult to implement in a rotorcraft having a tank that is remote from the engine to be fed. That configuration involves arranging pipes that follow sinuous paths in the rotorcraft in order to convey fuel under high pressure to an engine.
Document U.S. Pat. No. 3,259,066 describes an architecture for feeding an engine with the help of an ejector.
Document FR 2 949 352 discloses a system for monitoring a fluid filter for an aeroengine by comparing pressure measurements between upstream and downstream of the filter.
Document EP 0 806 318 discloses a fuel installation for a vehicle, the installation comprising a tank having a plurality of compartments with bottoms that are separate from one another. The installation has a transfer unit for taking fuel from the tank. The transfer unit is connected to a fuel-taking device present in a first compartment and in fluid flow communication with a second compartment via a suction duct. The transfer unit has orifices opening out into the first compartment.
Also known are Documents U.S. Pat. No. 2,953,156, US 2009/000844, U.S. Pat. No. 2,961,130, EP 0 278 755, and JP 2011 032898.